For the purpose of treating diseases of the human eye, such as, for example, keratotonus, and also for the purpose of eliminating mild forms of defective vision, above all myopia (short-sightedness), it is known to insert annular intrastromal corneal implants into a ring tunnel in the corneal tissue expressly created for them. In the state of the art, diverse configurations of such corneal ring implants are known which may differ, inter alia, by virtue of their peripheral length (closed ring, slotted ring, semi-ring or ring segment of different length), by virtue of their cross-sectional shape (e.g. circular, oval, triangular, hexagonal), by virtue of their cross-sectional progression in the peripheral direction (constant cross-section or variable cross-section, such as, for example, in the case of a sickle), by virtue of their material, and by virtue of whether they have a fixed cross-sectional shape and size or are adjustable in this respect. The implant may be in one piece or may consist of several separate ring segments (e.g. semi-rings) which are implanted in succession in the peripheral direction. A feature all these ring implants have in common is that they extend along a ring arc, customarily—but not necessarily—having a circular-arc-shaped curvature. By way of material, use is often made nowadays of PMMA (polymethyl methacrylate), in which connection other biocompatible materials have already been tried in the past and are, within the scope of the invention, by no means ruled out. The invention is generally applicable for arbitrary intrastromal corneal ring implants; there is no restriction to particular types.
In order that the ring implant can be inserted into the cornea, firstly a suitable ring tunnel (channel) with a peripheral length at least corresponding to the implant to be inserted has to be prepared in the stroma. In accordance with one method this may be accomplished by the operating surgeon manually with a suitable mechanical tool with which the stromal tissue layers can be separated from one another (e.g. spatula).
Laser-assisted cutting systems have recently become available with which it has become possible to place incisions and entire incision figures, in themselves of arbitrary two-dimensional or even three-dimensional configuration, in human ocular tissue and, above all, in the corneal issue and also in the lens tissue. The focused laser radiation employed in this connection is of ultra-short-pulse nature (with pulse durations within the femtosecond range) and has to have a wavelength in respect of which the tissue to be machined is transmissive. Wavelengths that are frequently employed lie within the near-infrared region (e.g. between 1 μm and 1.1 μm), but ultraviolet wavelengths above about 300 nm and also higher infrared wavelengths between approximately 1600 nm and 1700 nm are also possible for the preparation of incisions in the cornea or in the human lens.
For the generation of incisions by means of focused laser radiation in transparent material (transparent to the laser radiation), the so-called laser-induced optical breakthrough is utilised by way of physical effect. This results in a local vaporisation of the irradiated material, which is designated as photodisruption. The photodisruption is spatially restricted substantially to the area of the focus. By placing a plurality of such photodisruptions side by side, the most diverse incision figures can be generated.
The photodisruptive generation of incisions in the human cornea by means of ultra-short-pulse focused laser radiation has, for example, been proposed many times for the preparation of the flap in the course of a LASIK operation (LASIK: laser in-situ keratomileusis).